NEAR NET SHAPE FABRICATION OF PMN SCAFFOLDS Aylin

Afyon Kocatepe Üniversitesi
FEN BİLİMLERİ DERGİSİ
Özel Sayı
155-160
Afyon Kocatepe University
JOURNAL OF SCIENCE
NEAR NET SHAPE FABRICATION OF PMN SCAFFOLDS
Aylin ŞAKAR-DELİORMANLI1, Erdal ÇELİK2, Mehmet POLAT1
1
2
Izmir Institute of Technology, Chemical Engineering Department, Urla, Izmir/Turkey
Dokuz Eylul University, Metallurgy and Materials Engineering Department, Izmir/Turkey
ABSTRACT
Solid free form fabrication techniques are commonly employed in near net shaping of ceramics
especially in the manufacture of microelectronics. Lead magnesium niobate (PMN) is an important
relaxor ferroelectric material with perovksite structure used in the production of these multilayer
ceramic devices. In this study two dimensional PMN arrays and three dimensional periodic structures
were produced by robocasting which is a direct writing technique. Aqueous colloidal gels have been
formulated for use as ink in solid freeform fabrication process. Results showed that complex PMN
shapes having rod size down to 100 μm can be successfully produced by robocasting process to be
used in microelectronic applications.
Keywords: Shaping, Near Net Shape Fabrication, Robocasting, Perovskites, PMN
1. INTRODUCTION
Lead magnesium niobate is an important electrostrictive, relaxor ferroelectric material with perovskite
structure and commonly employed in the production of multilayer ceramic capacitors, actuators,
transducers, and motors [1-4]. Electrostrictive actuator is a category of transducers. In actuators it is
desirable to enhance the displacement while maintaining the load bearing capability [5]. This can be
achieved by manipulating the geometry of simple shape actuators into more complex geometries [6,
7]. Solid free form fabrication (SFF) techniques are able to produce 2-D or 3-D complex periodic
structures to use in these application areas [8-16]. Such techniques use computer-controlled robotics to
build three-dimensional components layer-by-layer [9, 12].
Robocasting is a free form fabrication technique and uses robotics to control layer wise deposition of
ceramic slurries for near net shape processing [13]. The technique utilizes highly concentrated ceramic
slurries with a small amount of organic binders [13, 14]. This direct write assembly technique is
capable of producing 3D periodic structures consisting rods with different geometrical shapes such as
cylindrical, square, hexagonal [15]. It has been utilized for the fabrication of periodic lattices of rods;
for piezocomposites [16], 3D microvascular networks and multi-material composites [17-19].
Previously, Lewis and co-workers and Cesarano et al. conducted detailed research on the robocasting
of alumina [13, 20], barium titanate [21, 22] and lead zirconate titanate [23] powders and produced
different geometries to be used in different applications [24]. The aim of this study is to produce 2D
arrays and 3D periodic PMN lattices to use in microelectronic devices and investigate their sintering
behavior and microstructural evolution.
Near Net Shape Fabrication of PMN Scaffolds
A. Şakar-Deliormanlı, E. Çelik, M. Polat
2. EXPERIMENTAL
2.1. Materials
Praxair Specialty Ceramics,
Lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 was provided by
Woodinville, WA, USA. Powder purity is 99.9% as reported by the manufacturer. Bulk density of the
powder is measured to be 7.967 g/cm3. Particle size, d50 of the powder was around 2.0 μm. Stable
colloidal suspensions were prepared by adding pol(yacrylic acid) to the system and gelation was
induced by adding poly(ethylene imine) PEI. Detailed description of the PMN ink design was reported
in our previous study [25].
2.2. Method
Periodic lattices were assembled using a robotic deposition apparatus (JL2000, Robocasting
Enterprises, Inc., Albuquerque, NM). Figure 1-a shows the picture of the robocaster that is used in the
study and Fig.1-b demonstrates the direct writing operation. The deposition was performed at a
volumetric flow rate of 7.5 mm/s. A constant pressure (between 10 – 100 psia) was applied to induce
the ink flow through the nozzle. The deposition process was carried out under a non-wetting oil to
prevent drying during assembly. Samples were dried at room temperature for 24 hours and at 80 ºC for
over night.
For the sintering of 3D periodic lattices following heating procedure were applied: The binder was
burned out by heating the samples at 1 ºC/min to 350 °C, 1 h dwell at 350 °C and 3°C/min to various
temperatures ranging from 1000 to 1300 °C. During heat treatment, alumina crucible system with
cover was used in order to prevent the lead loss from the samples. To provide a lead rich atmosphere,
lead zirconate (PbZrO3) powder bed was used inside the crucible.
(b)
(a)
Figure 1 . a) Pictures of the robocasting assembly -deposition module with camera, b) Picture showing the
printing process using 100 µm tip, Feature size 5mm.
Digital images of the 3D structures were taken by a Digital Camera (Kodak Easy Share Z650). Optical
images of the samples were obtained using a Stereo Microscope (Nikon, SMZ 2T, Light 2) with a
camera. Microstructures of the samples were observed using a scanning electron microscope, (Philips,
XL-30S FEG).
3. RESULTS AND DISCUSSION
3.1. Robocasting
In the study the PMN inks was robotically deposited onto a moving x-y stage yielding a 2-D pattern.
After a given layer was generated, the stage was increased in the z-direction and a second layer was
deposited. This process was repeated until the desired 3-D structure was
created. Depositions down to 100 µm were successfully performed. Figure 2 show the 3D lattices
produced by robocasting method. The pictures were taken just after deposition step therefore the
structures are still in oil reservoir. The radial lattice in Figure 2 has a cylindrical symmetry where the
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Near Net Shape Fabrication of PMN Scaffolds
A. Şakar-Deliormanlı, E. Çelik, M. Polat
top layer is an array of radial lines and the underlying layer is a series of concentric rings. The rings
appeared to maintain the arc shape during deposition.
Figure 2. Radial PMN structures produced by robocasting.
3.2. Optical Microscope Images and Microstructral Characterization
Two dimensional arrays and three dimensional assemblies consisting of a continuous spiral pattern
and a parallel array of rods were produced by robotic deposition using 100 µm tips, as shown in Figure
3-a and b. Similarly, pictures of three dimensional lattices are shown in the same figure (Fig 3, c-f).
Structures produced by PMN inks which contain adequate amount of PEI kept their shape during
deposition. Results showed that it was possible to create well shaped, uniform lattices having rod sizes
in the range of 100 µm to 500 µm using PMN ink in the presence of 0.04 to 0.06 mg/m2 PEI. At a PEI
concentration of 0.08 mg/m2 the yield stress of the PMN ink was too high therefore, at this
concentration the flow properties of the ink was poor. Above this value some difficulties were
observed in shape retention due to decrease in destabilization of PMN inks [25].
A
D
C
B
E
F
Figure 3. Pictures of two dimensional PMN arrays and three dimensional structures produced by robocasting a)
linear array, D: 100 μm b) radial array D: 100 μm c) square lattice, D: 200 μm d) square lattice, D: 100 μm e)
radial structure, D: 200 μm f) square lattice, D: 500 μm.
Figure 4 shows the surface view and the cross section of the dried samples produced by robotic
deposition. SEM micrographs reveal that samples contain some degree of porosity although they
contain negligible amount of binder. It is clear that samples sintered at 1180 °C did not show any
shape deformation during processing and kept their homogeneous structures. On the other hand
micrographs shown in Figure 4-d demonstrates that samples sintered at 1300 °C have a heterogeneous
microstructure and contain high amount of pyrochlore which is an undesired phase. The Figure 5
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Near Net Shape Fabrication of PMN Scaffolds
A. Şakar-Deliormanlı, E. Çelik, M. Polat
demonstrates the microstructures of the 3D PMN assemblies sintered at various temperatures.
Accordingly, samples sintered at 1000 °C have very high degree of porosity. At 1150 °C a decrease
was observed in porosity but samples still have relatively low density. On the other hand, sintering at
1180 °C produced a homogeneous and single phase structure having high densities and low porosity.
However, above that temperature (e.g. 1250 °C) significant grain growth was observed. Abnormal
grain growth in PMN with a broad distribution in grain sizes compromises the desired electrical
properties.
AB
CD
Figure 4. SEM micrographs of the structures produced by robotic deposition method a) green state b) after
sintering at 1150 °C for 2 hours c) 1180 °C for 2 hours d) 1300 °C for 2 hours.
A- 1000 °C
B- 1150 °C
C-1180 °C
D- 1250 °C
Figure 5. SEM micrographs of PMN samples showing the microstructural evolution as a function of sintering
temperature, sintering time 2 hours, heating rate 10 °C/min.
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500µm
Near Net Shape Fabrication of PMN Scaffolds
A. Şakar-Deliormanlı, E. Çelik, M. Polat
4. CONCLUSIONS
In this study 3D PMN lattices and radial structures were successfully manufactured by robotic
deposition method. Results showed that it is possible to produce complex PMN shapes having rod size
between 100 μm to 500 μm by this technique. The 3D assemblies sintered at 1180 °C for 2 hours have
low porosity, high density and homogeneous microstructures. For future studies our goal is to decrease
the rod size and manufacture more complex geometries to be used in microelectronic device
applications.
5. ACKNOWLEDGEMENTS
Authors would like to thank Prof.Dr.Jennifer A.Lewis, for use of the laboratory facilities and robotic
deposition device at University of Illinois at Urbana Champaign, USA. Contributions of Dr. John
Bukowski are gratefully acknowledged. The project is supported by The Scientific and Technological
Research Council of Turkey.
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